Security system

ABSTRACT

A security system which is suitable for use in a vehicle. The security system is a passive entry and passive re-mobilization system in which a security controller transmits a command signal LCs to a remote transponder, which replies with a LF response signal LRs. If an LF response signal is not received in response to the transmission of a command signal LCs, the security controller re-transmits the command signal, again as an LF command signal LCs. On determining that a command signal is a re-transmitted command signal, the remote transponder re-transmits the response signal as an UHF response signal URs.

This invention relates to security systems and in particular, but notexclusively, to a security system suitable for use in a vehicle.

BACKGROUND OF THE INVENTION

It is known to provide a protected area such as a building or a vehiclewith a security system which includes a passive entry feature and onesystem of this type is disclosed in EP 0 218 251. Such passive security,systems may, in addition or in the alternative, be applied to thestarting system off a vehicle to provide passive re-mobilisation of anengine. Interference from unrelated radio frequency sources can causeproblems for systems of this type.

It is also known to transmit signals from a portable transponder in morethan one frequency range. One arrangement of this type is shown in EP0440974, in which the transponder is used to perform different functionsin each range. In one range it sends security signals to the vehicle andin a second range it is operative to open remotely a garage door.

In circumstances where there is a disruption in the normal operatingconditions of some types of security system, it is possible to varyparameters of a signal from a remote transponder between successivetransmissions thereof. One system in which this occurs is disclosed inGB 2282252, and in this system parameters of the response signal arealtered on the basis of measured values, i.e. received signal strengthand/or measured battery power. It is a possible disadvantage of thisarrangement that it is necessary to provide means in the remotetransponder to determine the level of these values and this might addcomplexity and expense to the system.

SUMMARY OF THE INVENTION

It is an object 6f this invention to provide an improved securitysystem.

According to the invention there is provided a security systemcomprising a security control means and a remote transponder which arearranged in use to communicate in a range of channels using command andresponse signals transmitted in one or more of said channels, saidsecurity control means being arranged in use to transmit a said commandsignal in a said channel, said remote transponder being arranged in use,in response to a receipt of said command signal, to transmit a saidresponse signal initially in an initial said channel, said securitycontrol means being arranged, if it does not successfully receive a saidresponse signal, to re-transmit said command signal, wherein said remotetransponder is arranged to determine if a command signal is are-transmitted command signal and, if it determines that a said commandsignal is a re-transmitted said command signal, to re-transmit saidresponse signal in one or more of said channels which is/are differentto said initial channel.

Said remote transponder may be arranged to determine that a particularsaid command signal is a re-transmitted said command signal if itreceives, that particular said command signal within a time slot of apredetermined duration which commences on or after it has received anearlier said command signal.

Said security control means may be arranged, to modify said commandsignal before re-transmitting it, such that said modified re-transmittedcommand signal includes: information identifying it to said remotetransponder as a re-transniitted said command signal.

Said information may be indicative of which different said channel orchannels is or are to be used to re-transmit said response signal inreply to said re-transmitted command signal.

Said“channel of transmission of said command signal may operate in theLow Frequency band (LF).

Said initial channel of transmission of said response signal may operatein the Low Frequency band (LF).

In response to the detection of a re-transmitted said command signal,said response signal may be re-transmitted in a said channel whichoperates in the Ultra High Frequency band (UHF).

A re-transmission of said command signal may be performed in a saidchannel which operates in the Low Frequency band (LF).

Said remote transponder may further comprise a remote locking and/orunlocking means having a transmitter stage, wherein said re-transmissionof said response signal is performed using said transmitter stage and inat least one said channel used for at least one of said remote lockingor unlocking.

Said security control means may be arranged to receive said responsesignals in any of said range of channels and may further comprise ascanning means arranged in use to scan said range of channels in orderto determine in which said channel or channels said response signal orsaid re-transmitted response signal can be received.

Said remote transponder may be arranged, in response to a detection of are-transmitted said command signal, to re-transmit said response signalin a first different one of said channels and then in at least a seconddifferent one of said channels.

Said different channel or channels in which said response signal isre-transmitted may be substantially preset in said remote transponder.

Said security control means may be arranged to scan said range ofchannels for a said response signal or a said re-transmitted responsesignal by tuning itself to each of said channels and remaining tuned tothat said channel for long enough to detect the presence of data beingtransmitted in that said channel.

Said security control means may be arranged to re-transmit said commandsignal if it does not successfully receive a said response signal withina time slot of predetermined duration which commences on or after aninitial transmission of said command signal.

Said response signal may be produced by modulating said command signaland said modulation may be in the order of 2.5% of said command signal.

The security system may comprise the security system of a vehicle.

The invention also provides a method of controlling a security system,the security system comprising a security control means and a remotetransponder which are arranged in use to communicate in a range ofchannels using command and response signals transmitted in one or moreof said channels, said security control means being arranged in use totransmit a said command signal in a said channel, said remotetransponder being arranged in use, in response to a receipt of saidcommand signal, to transmit a said response signal initially in aninitial said channel, said security control means being arranged, if itdoes not successfully receive a said response signal, to re-transmitsaid command signal, the method including the steps of:

a) determining if a command signal is a re-transmitted command signal;and

b) re-transmitting, if a said command signal is determined to be are-transmitted said command signal, said response signal in one or moreof said channels which is/are different to said initial channel.

The method may include determining that a particular said command signalis a re-transmitted said command signal if said remote transponderreceives that particular said command signal within a time slot of apredetermined duration which commences on or after it has received anearlier said command signal.

The method may include modifying said command signal beforere-transmitting it, such that said modified re-transmitted commandsignal includes information identifying it to said remote transponder asa re-transmitted said command signal.

The method may include indicating in said information which differentsaid channel or channels is or are to be used to re-transmit saidresponse signal in reply to said re-transmitted command signal.

The method may include transmitting said command signal in a saidchannel which operates in the Low Frequency band (LF).

The method may include transmitting said initial response signal in asaid channel which operates in the Low Frequency band (LF).

The method may include, in response to the detection of a re-transmittedsaid command signal, re-transmitting said response signal in a saidchannel which operates in the Ultra High Frequency band (UHF).

The method may include re-transmitting said command signal in a saidchannel which operates in the Low Frequency band (LF).

Said remote transponder may further comprise a remote locking and/or,unlocking means having a transmitter stage and the method may includere-transmitting said response signal using said transmitter stage and inat least one said channel used for at least one of said remote lockingor unlocking.

Said security control means may be capable of receiving a said responsesignal in any of said channels and may further comprise a scanning meanscapable of scanning said channels, and the method may include scanningsaid range of channels in order to determine in which said channel orchannels said response signal or said re-transmitted response signal canbe received.

The method may include, in response to a detection of a re-transmittedsaid command signal, re-transmitting said response signal in a firstdifferent one of said channels and then in at least a second differentone of said channels.

The method may include substantially presetting in said remotetransponder said channel or channels in which said response signal isre-transmitted.

The method may include scanning said range of channels for a saidresponse signal or a said re-transmitted response signal by tuning saidsecurity control means to each of said channels and remaining tuned tothat said channel for long enough to detect the presence of data beingtransmitted in that said channel.

The method may include re-transmitting said command signal if saidsecurity control means does not successfully receive a said responsesignal within a time slot of predetermined duration which commences onor after an initial transmission of said command signal.

The method may include producing said response signal by modulating saidcommand signal and may include modulating said command signal by about2.5%.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a security system according to theinvention; and

FIG. 2 is a flow diagram of a method of operating the security system ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the figures, a security system for a vehicle (not shownfurther) comprises a passive entry and passive re-mobilisation systemand includes a security control means in the form of a securitycontroller 10 which is fitted to the vehicle and which is arranged inuse to detect the likely presence of a smart key 12 in the proximity ofthe vehicle.

For passive entry purposes, the security controller 10 is arranged todetect the presence of the key 12 by monitoring a micro-switch which isincluded in a door handle assembly (neither shown) and which switch isactivated when the door handle is operated. For passive re-mobilisationpurposes, a further micro-switch is included in an ignition switchassembly (neither shown), that micro-switch being activated when the key12 is inserted so as to indicate to the security controller 10 thepresence of the key 12 in the ignition/starter switch.

The key 12 includes a remote transponder 14 which itself includes acontrol logic 16 which controls two transponder stages, a Low Frequency(LF) transmitter/receiver stage L_(k) and an Ultra High Frequency (UHF)transmitter stage U_(k).

Also included in the remote transponder 14 is a lock switch S1 and anunlock switch S2 which are push-button user operable and are arranged inuse to transmit “lock vehicle/engine immobilise” and “unlockvehicle/engine re-mobilise” commands respectively to the securitycontroller 10 by using the UHF transmitter U_(k) to transmit UHF signalsU_(s). The switches S1, S2 can be used for remote implementation oflock-immobilise/unlock-remobilise instructions.

The security controller 10 includes an LF transmitter / receiver stageL_(c) and an UHF receiver stage U_(c) which are arranged to communicatewith the transmitter/receiver stage L_(k) and the UHF transmitter U_(k)of the remote transponder 14 respectively.

The operation of the invention is now described with specific, but notlimiting, reference to passive remobilisation.

When the security controller 10 detects the likely presence of theremote transponder 14 in the immediate vicinity of the ignition switch,it 10 transmits a command signal LCs in the LF band to the remotetransponder 14 at about 125 kHz, which it 14 receives though the LFtransmitter/receiver stage L_(k). The control logic 16 decodes thereceived command signal LCs and, if it is recognised as a genuinecommand signal LCs, the control logic 16 causes the transmitter receiverstage L_(k) to transmit in LF, also at about 125 kHz, a response signalLRs back to the security controller 10 which it 10 receives through itsLF transmitter/receiver stage L_(c). In this manner, the LF responsesignal LRs is initially transmitted in a predetermined channel which issubstantially fixed in the transponder 14.

The response signal LRs is received by tuning the receiver stage L_(k)to the frequency of the initial response channel or by scanning throughthe channels, in similar fashion to that outlined below for UHFchannels, until the channel in which it LRs can be received isdetermined. The reader is also referred to our co-pending application GB9827361.8 (which is published as GB 2332548A) for further details of asimilar such scanning operation.

On receipt of a valid LF response signal LRs, the security controller 10performs one or more predetermined security functions, for exampleallowing passive” re-mobilisation of and immobilised engine so as toallow starting of the vehicle. The security controller 10 performs thepassive re-mobilisation function in substantially the same manner itwould an equivalent “unlock/engine re-mobilise” function of theappropriate push-button switch S2.

If the security controller 10 does not receive the LF response signalLRs within a time slot of predetermined duration which commences afterthe transmission of the command signal LCs, the security controller 10is arranged to determine (e.g. assume) that the command signal LCs haseither not, been received by the remote transponder 14 or that the LFresponse signal LRs has been corrupted. If the LF response signal LRs isnot received within the time slot, the security controller 10 isarranged to re-transmit the command signal LCs, still in the LF band.

The controller 10 transmits its command signals LCs on LF alone becausethey are only transmitted when a transponder 14, is likely to be presentin the immediate vicinity of the ignition switch and can be transmittedat quite high power, for example the command signal LCs may betransmitted at about 40 V peak-to-peak, and this is considered to beadequate to ensure sufficiently reliable reception by the transponder14.

In this embodiment, the transponder 14 produces its LF response signalLRs by loading a received command signal LCs and modulating it by about2.5%. Although it is unlikely that a command signal LCs will beinterfered with to the extent that it is not received because of itspower, it can be seen that it might be possible to suffer sufficientinterference to the response signal LRs to swamp the about 2.5%modulation.

If the remote transponder 14 receives a second command signal LCs, forexample during a predetermined time slot following its first receipt ofthe command signal LCs, the transponder control logic 16 is arranged totreat such a further command signal LCs as a re-transmitted commandsignal. The remote transponder 14 is arranged to reply to are-transmitted command signal with a UHF response signal URs using itsUHF transmitter U_(k).

In the UHF band, the transmitter U_(k) operates at a bandwidth in theorder of 480 kHz with 32 channels, which are each about 15 kHz wide andcentred around a frequency of about 433 MHz. When responding to are-transmitted command signal, the remote transponder 14 is arranged totransmit the UHF response signal URs in two of the 36 channels, first inone channel and then in a second one. Using successive transmissionsmeans that harmonics are not generated and the problems associated withfrequency shift keying (FSK) do not occur, which in turn means thatchannel spacing is not restricted as it would be if FSK were to be used.

The channels used for the UHF response signal URs are fixed in theremote transponder 14 during manufacture by the inclusion of a pair ofsurface acoustic wave resonators (SAWs) (not shown separately) which areincluded in the control logic 16. The SAWs are connected in parallel andarranged to be capable of being switched on independently andselectively.

To send the UHF response signal URs, one of the SAWs is turned on usinga pin diode and causes the UHF response signal URs to be transmitted inthe channel in which its resonant frequency falls. This SAW is thenturned off and the other SAW is turned on, again using a pin diode. TheUHF response signal URs is then transmitted in the channel in which theresonant frequency of the second SAW falls. The second SAW is thenturned off and transmission of the UHF response signal URs for thatreceipt of the re-transmitted command signal is complete.

The SAWs are selected, such that their nominal resonant frequenciesresult in the transmission channels of the UHF response signal URshaving a spacing of at least three or four UHF channels between them, ormore preferably a spacing of about at least 100 kHz.

The SAWs frequencies cannot be guaranteed to be fixed, as their nominalresonant frequency will drift with time and variations in temperature.Because of this, it is not possible to specify with absolute certaintywhich of the channels will be used to transmit the UHF response signalURs and the UHF receiver stage U_(c) must therefore scan all thechannels to find the one or more than one in which the UHF responsesignal URs can be detected.

To scan across all the channels, the UHF receiver stage U_(c) scans thechannels by tuning itself to each one in turn and waiting in eachchannel long enough to detect a UHF response signal URs in an expectedformat. One example of the detection of a correct format can be shown byproviding the UHF response signal URs with a long preamble of logic “1”bits so as to make it easy to find and then keeping the UHF receiverstage U_(c) tuned into each channel long enough to detect a recognisableportion of this preamble.

When the UHF receiver stage U_(c) has found a channel in which the UHFresponse signal URs from the remote transponder 14 appears to bepresent, it uses an automatic frequency control (AFC) process to centreon the transmitted frequency and stays there until the data becomesimplausible or until a time out has expired.

If the data becomes implausible and under-runs or over-runs, that eventis noted by incrementing a register value for that channel by aconstant. A different constant is used when incrementing the registerfor each of an under-run and an over-run. The constants are not equaland the constant used for an over-run is the greater because an over-runusually indicates that the signal has been subjected to interference.

If the register value for a given channel reaches one of a series ofthresholds, the UHF receiver stage U_(c) is arranged to assume thatthere is an UHF interfering signal which is causing it to waste timelooking for the UHF response signal URs in that channel at that time.When this happens, the UHF receiver stage U_(c) first reduces thesensitivity of its receiver portion for the affected channel and does soin graduated steps (for example 4, 8, 12 dB). If the register valueexceeds a preset maximum despite the reduction or reductions insensitivity, the UHF receiver stage U_(c) will finally shut down forthat channel or channels which is/are then missed out completely. At theend of each scan, the register value for each channel is decremented.

A different way of deciding that the data is implausible would be tostay centred onto a string of detected data for a predetermined timeperiod, the expiry of which could be pre-programmed into the UHFreceiver stage U_(c) as indicative that the data being received does notcomprise a plausible signal. For example, such a time-out could be setto correspond to an over-run condition.

When the UHF receiver stage U_(c) has detected a plausible responsesignal it is passed on to the security controller 10 and is followed bya message which identifies the channel in which the UHF response signalURs was detected. The security controller 10 uses this information aboutthat channel to make a comparison with the other channel in which theUHF response signal URs was detected and to judge whether the channelspacing is sufficient to indicate a valid UHF response signal URs or onewhich has been interfered with. If the UHF response signal URs isdetected in only one channel, then information about which channel thatmight be is of limited use, except for example to verify that the UHFresponse signal URs was detected in a valid channel.

By sending out the UHF response signal URs on first one channel and thenon another one, instead of for example both at the same time, thelikelihood of generating undesirable harmonics is significantly reduced.

In a modification to the invention, the remote transponder 14 operatesin UHF at around a substantially fixed frequency in the order of 433 MHzand is arranged to transmit the UHF response signal URs at thatsubstantially fixed frequency. This provides a simplified version of theinvention which may prove cheaper to produce and would be adequate foruse in markets where there is not much crime.

The use of a bandwidth of about 480 kHz around a central frequency ofabout 433 MHz and which is divided into 32 channels each being about 15kHz wide, provides security against the use of a relaytransmitter/receiver and is particularly useful in markets where thereis a high risk of vehicle crime.

By transmitting the UHF response signal URs on more than one channel,the user gains an advantage in that a channel spacing of three or fourcan be used to transmit the signals closer to each other than would beeasy to detect using a commercially available UHF receiver of reasonablecost and which might have a bandwidth of, for example, 500 kHz. In thismanner, a thief trying to use a code scanner based on such technologywould find it more difficult to collect the UHF response signal URs andto re-transmit it. The UHF response signals URs would appear as a singlesignal and that is what would be re-transmitted after capture orrelaying. Because the thief s re-transmitted or relayed signal is nottwo signals of narrow bandwidth which are close to each other, thesecurity controller 10 could determine that such a signal was false orcorrupted. Channel spacing of three or four also provides sufficientspacing to take account of the tolerances and drift of any SAW used. Itwould be even better in this respect to set the channel spacing at 100kHz or more.

In a modification to the invention, the security controller is arrangedto modify the command signal LCs before re-transmitting it so as tospecify to the transponder 14 that it 14 should re-transmit its responsesignal as a UHF response signal URs. In the version of the invention inwhich the UHF transmitter stage U_(k) operates by transmitting the UHFresponse signal URs in more than one of a plurality of UHF channels, itwould also be possible to modify the command signal LCs further beforere-transmitting it, so as to specify in which one or more of those UHFchannels the transponder 14 should transmit its UHF response signal URs.

While the invention has been described with particular reference topassive re-mobilisation, it will be appreciated that its scope is not solimited. The apparatus and method are also suitable for the passiveentry feature. For example, a passive entry command signal could beinitiated on operating a door handle. It would also be possible to usethe same or different command and response signal for each of passiveentry and passive remobilisation.

What is claimed is:
 1. A security system comprising a security controlmeans and a remote transponder which are arranged in use to communicatein a range of channels using command and response signals transmitted inone or more of said channels, said security control means being arrangedin use to transmit a said command signal in a said channel, said remotetransponder being arranged in use, in response to a receipt of saidcommand signal, to transmit a said response signal initially in aninitial said channel, said security control means being arranged, if itdoes not successfully receive a said response signal, to re-transmitsaid command signal, wherein said remote transponder is arranged todetermine if a command signal is a re-transmitted command signal and, ifit determines that a said command signal is a re-transmitted saidcommand signal, to re-transmit said response signal in one or more ofsaid channels which is/are different to said initial channel.
 2. Asecurity system according to claim 1, wherein said remote transponder isarranged to determine that a particular said command signal is are-transmitted said command signal if it receives that particular saidcommand signal within a time slot of a predetermined duration whichcommences on or after it has received an earlier said command signal. 3.A security system according to claim 1, wherein said security controlmeans is arranged to modify said command signal before re-transmittingit, such that said modified re-transmitted command signal includesinformation identifying it to said remote transponder as are-transmitted said command signal.
 4. A security system according toclaim 2, wherein said information is indicative of which different saidchannel or channels is or are to be used to re-transmit said responsesignal in reply to said re-transmitted command signal.
 5. A securitysystem according to claim 1, wherein said channel of transmission ofsaid command signal operates in the Low Frequency band (LF).
 6. Asecurity system according to claim 1, wherein said initial channel oftransmission of said response signal operates in the Low Frequency band(LF).
 7. A security system according to claim 1, wherein, in response tothe detection of a re-transmitted said command signal, said responsesignal is re-transmitted in a said channel which operates in the UltraHigh Frequency band (UHF).
 8. A security system according to claim 1,wherein a re-transmission of said command signal is performed in a saidchannel which operates in the Low Frequency band (LF).
 9. A securitysystem according to claim 1, wherein said remote transponder furthercomprising a remote locking and/or unlocking means having a transmitterstage, wherein said re-transmission of said response signal is performedusing said transmitter stage and in at least one said channel used forat least one of said remote locking or unlocking.
 10. A security systemaccording to claim 1, wherein said security control means is arranged toreceive said response signals in any of said range of channels andfurther comprises a scanning means arranged in use to scan said range ofchannels in order to determine in which said channel or channels saidresponse signal or said re-transmitted response signal can be received.11. A security system according to claim 10, wherein said remotetransponder is arranged, in response to a detection of a re-transmittedsaid command signal, to re-transmit said response signal in a firstdifferent one of said channels and then in at least a second differentone of said channels.
 12. A security system according to claim 10,wherein said security control means is arranged to scan said range ofchannels for a said response signal or a said re-transmitted responsesignal by tuning itself to each of said channels and remaining tuned tothat said channel for long enough to detect the presence of data beingtransmitted in that said channel.
 13. A security system according toclaim 10, wherein said security control means is arranged to scan saidrange of channels for a said response signal or a said re-transmittedresponse signal by tuning itself to each of said channels and remainingtuned to that said channel for long enough to detect the presence ofdata; being transmitted in that said channel.
 14. A security systemaccording to claim 1, wherein said security control means is arranged tore-transmit said command signal if it does not successfully receive asaid response signal within a time slot of predetermined duration whichcommences on or after an initial transmission of said command signal.15. A security system according to claim 1, wherein said response signalis produced by modulating said command signal.
 16. A security systemaccording to claim 15, wherein said modulation being in the order of2.5% of said command signal.
 17. A security system according to claim 1,comprising the security system of a vehicle.
 18. A method of controllinga security system, the security system comprising a security controlmeans and a remote transponder which are arranged in use to communicatein a range of channels using command and response signals transmitted inone or more of said channels, said security control means being arrangedin use to transmit a said command signal in a said channel, said remotetransponder being arranged in use, in response to a receipt of saidcommand signal, to transmit a said response signal initially in aninitial said channel, said security control means being arranged, if itdoes not successfully receive a said response signal, to re-transmitsaid command signal, the method including the steps of: a) determiningif a command signal is a re-transmitted command signal; and b)re-transmitting, if a said command signal is determined to be are-transmitted said command signal, said response signal in one or moreof said channels which is/are different to, said initial channel.
 19. Amethod according to claim 18, including determining that a particularsaid command signal is a re-transmitted said command signal if saidremote transponder receives that particular said command signal within atime slot of a predetermined duration which commences on or after it hasreceived an earlier said command signal.
 20. A method according to claim18, including modifying said command signal before re-transmitting it,such that said modified re-transmitted command signal includesinformation identifying it to said remote transponder as are-transmitted said command signal.
 21. A method according to claim 20,including indicating in said information which different said channel orchannels is or are to be used to re-transmit said response signal inreply to said re-transmitted command signal.
 22. A method according toclaim 18, including transmitting said command signal in a said channelwhich operates in the Low Frequency band (LF).
 23. A method according toclaim 18, including transmitting said initial response signal in a saidchannel which operates in the Low Frequency band (LF).
 24. A methodaccording to claim 18, including, in response to the detection of are-transmitted said command signal, re-transmitting said response signalin a said channel which operates in the Ultra High Frequency band (UHF).25. A method according to claim 18, including re-transmitting saidcommand signal in a said channel which operates in the Low Frequencyband (LF).
 26. A method according to claim 18, said remote transponderfurther comprising a remote locking and/or unlocking means having atransmitter stage and the method including re-transmitting said responsesignal using said transmitter stage and in at least one said channelused for at least one of said remote locking or unlocking.
 27. A methodaccording to claim 18, said security control means being capable ofreceiving a said response signal in any of said channels and furthercomprising a scanning means capable of scanning said channels, themethod including scanning said range of channels in order to determinein which said channel or channels said response signal or saidre-transmitted response signal can be received.
 28. A method toaccording to claim 27, including, in response to a detection of are-transmitted said command signal, re-transmitting said response signalin a first different one of said channels and then in at least a seconddifferent one of said channels.
 29. A method according to claim 27,including substantially presetting in said remote transponder saidchannel or channels in which said response signal is re-transmitted. 30.A method according to claim 27, including scanning said range ofchannels for a said response signal or a said re-transmitted responsesignal by tuning said security control means to each of said channelsand remaining tuned to that said channel for long enough to detect thepresence of data being transmitted in that said channel.
 31. A methodaccording to claim 18, including re-transmitting said command signal ifsaid security control means does not successfully receive a saidresponse signal within a time slot of predetermined duration whichcommences on or after an initial transmission of said command signal.32. A method according to claim 18, including producing said responsesignal by modulating said command signal.
 33. A method according toclaim 32, including modulating said command signal by about 2.5%.